As shown in the Japanese laying-open patent publication sho No. 62-167456(1987), the well known conductivity detector has two kinds of correction methods for compensating background noise of elute. One method performs correction of A.C. signal at input stage and the other correction of D.C. signal after rectification stage. In the correction operation, the D.C. signal correction is carried out at first, and if the background noise level is too high to be eliminated, the A.C. signal correction is performed until the noise level becomes low enough to be eliminated by D.C. signal correction.
Owing to the time delay of the rectifying circuit, the above prior art has a time difference between the correction signal for eliminating the background noise and a zero detection signal for judging completion of the correction operation. Accordingly, it requires a lot of time to repeat the correction process until the background noise becomes zero.